1. Field of the Invention
The present invention relates to a process for the preparation of catalysts based on MFI-type zeolite in spheroidal form.
2. Description of the Background
More specifically, the present invention relates to a process for the preparation of catalysts based on MFI-type zeolite active in rearrangement reactions of oximes to amides and suitable for use in gas phase in fluid bed and moving bed reactors.
The invention also relates to the catalysts obtained by means of the above processes and to the processes in which they are used.
MFI-type zeolites, in particular those with a high silica/alumina ratio (U.S. Pat. No. 4,359,421) and, more generally, those with a low content of trivalent heteroelements (patent EP 242,960), are known in literature as basic materials for the preparation of catalysts which can be used in numerous reactions and in particular in rearrangement reactions of oximes to amides; among these, particular importance is given to reactions carried out in gaseous phase. For example, patent EP 234,088 describes a method for the preparation of ε-caprolactam consisting in putting cyclohexanone-oxime, in the gaseous state, in contact with crystalline alumino-silicates having well-defined physico-chemical characteristics and preformed in the form of granules (24÷48 mesh).
These materials, however, consisting of the active phase only, have limited possibilities of being used in industrial reactors; if, in fact, fluid bed or moving bed reactors are to be used for the catalytic process, the catalysts should preferably have the form of microspheres, with an average diameter of 30÷100 μm and characterized by a high resistance to interparticle attrition and attrition against the walls of the reactors; if, on the other hand, fixed bed reactors are used, the catalysts should have the typical forms for this technology (spheres, tablets, etc.), with dimensions in the order of several millimeters and characterized by a good loading resistance.
The cohesion between the individual particles of the micro-crystalline zeolitic material is generally poor, and consequently the resistance to attrition and loading is usually obtained by combining the zeolitic material with compounds of an inorganic nature (ligands) in the forming phase.
Catalysts based on zeolites, suitable for use in fluid bed or moving bed reactors and with the specific characteristics mentioned above, are widely described in the known art and are mainly used in catalytic cracking processes (FCC, Fluid Catalytic Cracking catalysts).
In the forming of the above catalysts, normally effected with the known spray-drying technique, when microspheres with a diameter <100 μm are required, silicas and aluminum oxides in the colloidal state or silico-aluminates are used to give the microspheres a higher resistance to attrition. The use of these ligands in spherulization processes of zeolitic materials of the MFI type can however, in some applications, jeopardize their catalytic performances, as these ligands are not entirely inactive in the above reactions.
It is known, for example, that in the catalytic rear-rangement reaction of oximes to amides, the presence of ligands significantly jeopardizes the selectivity of the zeolitic catalyst and the deterioration of the catalytic performances caused by the formation of organic pitches [Catalysis Letters 17 (1993), 139-140; Catalysis Today 38 (1997), 249-253].
To overcome this problem, patent EP 576,295 suggests, for example, that the zeolitic material in spherical form be preformed by means of spray-drying without any addition of ligands and that the microspheres be subjected, in a subsequent process phase, to thermal treatment in water to increase their hardness.
In a more recent patent (EP 1,002,577), on the other hand, the use of silica ligands is suggested, which, when synthesized by the acid hydrolysis of silicon alkoxides, are practically inert in rearrangement reactions of oximes to amides. With these ligands and by means of a forming process via emulsion, catalysts are obtained in the form of microspheres characterized by a content of silica ligand, expressed as SiO2, equal to or higher than 30% by weight and by a resistance to attrition suitable for carrying out the rearrangement reaction in fluid bed or moving bed reactors.
Although the silica ligand is practically inert, the high percentage of SiO2 in these catalysts tends however to jeopardize the catalytic performances of the active phase. Furthermore, the forming process described proves to be complex, costly and difficult to develop on a technologically significant scale.